SQL
Structured Query Language
SQL is an ANSI (American National Standards Institute)
standard computer language for accessing and
manipulating database systems. SQL statements are used
to retrieve and update data in a database. SQL works with
database programs like MS Access, DB2, Informix, MS SQL
Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the
SQL language, but to be in compliance with the ANSI
standard, they must support the same major keywords in
a similar manner (such as SELECT, UPDATE, DELETE,
INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their
own proprietary extensions in addition to the SQL
standard!
SQL is a Standard - BUT
SQL Database Tables
A database most often contains one or more tables. Each table is
identified by a name (e.g. "Customers" or "Orders"). Tables contain
records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
SQL Queries

LastName
Hansen
Svendson
Pettersen
SQL Data Manipulation
Language (DML)
SQL (Structured Query Language) is a syntax for executing
queries. But the SQL language also includes a syntax to
update, insert, and delete records.
These query and update commands together form the Data
Manipulation Language (DML) part of SQL:
•
SELECT - extracts data from a database table
•
UPDATE - updates data in a database table
•
DELETE - deletes data from a database table
•
INSERT INTO - inserts new data into a database table
SQL Data Definition Language
(DDL)
The Data Definition Language (DDL) part of SQL permits
database tables to be created or deleted. We can also define
indexes (keys), specify links between tables, and impose
constraints between database tables.
The most important DDL statements in SQL are:
•
CREATE TABLE - creates a new database table
•ALTER TABLE - alters (changes) a database table
•

DROP TABLE - deletes a database table
•
CREATE INDEX - creates an index (search key)
•
DROP INDEX - deletes an index
SQL The SELECT Statement
The SELECT statement is used to select data from a table. The
tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s)
FROM table_name
To select the columns named "LastName" and "FirstName",
use a SELECT statement like this:
SELECT LastName, FirstName FROM Persons
Persons
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
výsledok
LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
To select all columns from the "Persons" table, use a *
symbol instead of column names, like this:
SELECT * FROM Persons
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes

Pettersen Kari Storgt 20 Stavanger
Select All Columns
The result from a SQL query is stored in a result-set. Most database
software systems allow navigation of the result set with
programming functions, like: Move-To-First-Record, Get-Record-
Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To
learn about accessing data with function calls, please visit our ADO
tutorial.
The Result Set
Semicolon is the standard way to separate each SQL statement in
database systems that allow more than one SQL statement to be
executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is
this necessary? We are using MS Access and SQL Server 2000
and we do not have to put a semicolon after each SQL statement,
but some database programs force you to use it.
Semicolon after SQL Statements?
The DISTINCT keyword is used to return only distinct (different)
values.
The SELECT statement returns information from table columns.
But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the
SELECT statement:
Syntax
SELECT DISTINCT column_name(s)
FROM table_name
The SELECT DISTINCT Statement
To select ALL values from the column named "Company" we use
a SELECT statement like this:

SELECT Company FROM Orders
Orders
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Company
Sega
W3Schools
Trio
W3Schools
Using the DISTINCT keyword
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named
"Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Company
Sega
W3Schools
Trio
Orders
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Select All Columns
The WHERE clause is used to specify a selection criterion.
The WHERE Clause

To conditionally select data from a table, a WHERE clause can
be added to the SELECT statement.
Syntax
SELECT column FROM table
WHERE column operator value
With the WHERE clause, the following operators can be
used:
Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
Note: In some versions of SQL the <> operator may be written as !=
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a
WHERE clause to the SELECT statement:
SELECT * FROM Persons
WHERE City='Sandnes'
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen Kari Storgt 20 Stavanger 1960
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978

Svendson Stale Kaivn 18 Sandnes 1980
Using Quotes
Note that we have used single quotes around the conditional
values in the examples.
SQL uses single quotes around text values (most database
systems will also accept double quotes). Numeric values should
not be enclosed in quotes.
For text values:
This is correct:
SELECT * FROM Persons WHERE FirstName='Tove'
This is wrong:
SELECT * FROM Persons WHERE FirstName=Tove
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in
a column.
Syntax
SELECT column FROM table
WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters
in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names
that start with an 'O':
SELECT * FROM Persons
WHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names
that end with an 'a':
SELECT * FROM Persons
WHERE FirstName LIKE '%a'
Using LIKE 2

The following SQL statement will return persons with first names
that contain the pattern 'la':
SELECT * FROM Persons
WHERE FirstName LIKE '%la%'
SQL The INSERT INTO
Statement
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into
a table.
Syntax
INSERT INTO table_name
VALUES (value1, value2, )
You can also specify the columns for which you want to insert
data:
INSERT INTO table_name (column1, column2, )
VALUES (value1, value2, )
Insert a New Row
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
And this SQL statement:
INSERT INTO Persons
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Insert Data in Specified Columns
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
And This SQL statement:

INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Rasmussen Storgt 67